1. Field of the Invention
The present invention relates to a microresonator having a double-ended tuning fork configuration and, more particularly, to such a microresonator having a preferred geometry and electrode configuration to optimize resonator performance when operating in the fundamental and first and second overtone flexure modes.
2. Description of the Prior Art
Piezoelectric resonators have been known since at least the early 1940's when work completed at Bell Laboratories was reported. Over the years, such devices have been made smaller and smaller to facilitate their use as accurate signal sources, such as in time measuring devices. Microresonators are now commonly made from piezoelectric materials such as quartz, as shown in U.S. Pat. Nos. 3,488,530, 3,969,640 and 3,766,616, issued to Juergen H. Staudte. The application of microphotolithographic processes developed in the integrated electronic circuit industry has permitted high volume manufacture of microresonators small enough for incorporation into ladie's wristwatches.
One common configuration of a quartz crystal microresonator is the so-called double-ended tuning fork which may be likened to two tuning forks secured end to end. Double ended tuning forks have been proposed for a variety of uses, including use in a beat frequency oscillator (U.S. Pat. No. 2,854,581), use as an ultrasonic transducer with feedback control (U.S. Pat. No. 3,148,289), use as an accelerometer element (U.S. Pat. No. 3,238,789), and use as a force transducer (U.S. Pat. Nos. 4,215,570; 4,299,122; 4,372,173; and 4,321,500).
Using photolithographic chemical etching manufacturing techniques, better control in the precision of crystal geometrical dimensions is provided. These techniques also create more flexibility for a crystal designer to use more complex crystal geometries to optimize the double-ended tuning fork quartz crystal parameters. However, while double-ended tuning fork quartz crystals have been known and are potentially useful in many sensor applications, there are no published detailed designs which optimize resonator parameters.